My invention comprises improvements in a continuously variable transmission for automotive vehicles having a throttle-controlled internal combustion engine. It is for use with infinitely variable transmissions having a hydrokinetic torque converter between the engine and the torque input elements of the infinitely variable ratio torque transfer members.
Examples of infinitely variable transmissions capable of incorporating the improvements of the invention are the infinitely variable transmissions shown in U.S. Pat. Nos. 4,876,920 and 4,583,423.
Infinitely variable transmissions such as those disclosed in the references mentioned above are capable of allowing the engine and the transmission to transfer torque from the engine to the traction wheels of the vehicle with a continuously variable ratio such that the engine will operate at a target speed that will achieve its most efficient specific fuel consumption. Improved fuel efficiency can be achieved throughout the entire range of vehicle speed by allowing the converter turbine speed to be controlled to a target value that will cause the engine speed to operate at a value for any given engine throttle setting that will achieve the minimum brake specific fuel consumption.
Copending U.S. patent application Ser. No. 028,019, filed Mar. 8, 1993, entitled "Continuously Variable Transmission" discloses a control system for making control adjustments that will allow the engine speed to increase upon advancement of the engine throttle in such a way that driveability is improved. The engine speed changes in a manner that is consistent with an increasing vehicle speed as torque demand increases. That copending application is assigned to the assignee of my present invention.
If the torque converter used with the infinitely variable transmission includes a lockup clutch, the turbine speed during engagement of the lockup clutch will force changes in engine speed. If the transmission is calibrated properly, the engine speed then will achieve a target value that will correspond to the speed consistent with minimum brake specific fuel consumption at the particular throttle setting that is commanded by the vehicle operator. If the clutch is engaged rapidly, the change in engine speed will result in a lack of smoothness and a noticeable shift sensation during the time the lockup clutch is increasing in torque transmitting capacity. This makes necessary a balancing of the driveline smoothness considerations with fuel economy considerations.
The improvement of the present invention is adaptable to an infinitely variable transmission that is controlled by a microprocessor with closed loop control of the speed ratio and torque converter slip. When the torque converter clutch is locked up, turbine speed is the same as the engine speed. By appropriately calibrating the rate of engagement of the clutch and the change in speed ratio during the time that the engine speed is decreasing, it is possible to achieve an unperceived decrease of the engine speed with a minimum energy absorption by the clutch. This is done with a coordinated speed ratio control strategy and converter clutch control strategy. The time of engagement required by the torque converter clutch is significantly lower than the total time required to decrease engine speed to the desired or optimum engine speed for an engine throttle setting where minimum fuel consumption is achieved.
My present invention includes a control strategy and a control system for implementing that strategy so that an early lockup of the torque converter clutch is achieved.